1. Field
This invention relates to proximity detectors. More particularly, this invention relates to proximity detectors of the type to detect the proximity of a ferromagnetic object and to proximity detectors for use with ski lifts for detecting a derope condition of the cable of the ski lift system.
2. State of the Art
Switches or devices to determine the proximity of an object have long been known. Such devices operate on a variety of principles including mechanical, electrical, magnetic and combinations thereof.
In recent years, magnetic proximity detectors have achieved some acceptance. U.S. Pat. No. 3,562,603 (Smith) discloses such a detector. In particular, it discloses a detector having a permanent magnet and a reed switch. The switch is operable by the magnetic flux field associated with the permanent magnet. A variety of other proximity switches are also commercially available. For example, the Honeywell Corporation of Minneapolis, Minn. has available model "4FR" and model "6FR" proximity switches which operate as a single-pole single-throw switch using magnets in association with a magnetically operated reed switch.
In practice, the above switches and others heretofore known have positioned permanent magnets with the axis and poles of their flux fields oriented substantially or essentially parallel to the front of the detector. When a ferromagnetic object approaches the detector front, the magnetic field of the permanent magnet is modified in such a way as to cause the reed switch to close or open. The permanent magnets used are typically elongated bar magnets oriented parallel to the face, front or detecting surface of the detector. In the case of the 4FR and 6FR switches made by the Honeywell Corporation, one elongated bar magnet is positioned along and parallel to the sensing face. A second permanent magnet is spaced away from the first. The reed switch is in between the two magnets. Such an arrangement exposes the detector to influence from stray magnetic fields and to influence from magnetic objects behind and around the detector. Indeed, such detectors may not be reliable when used in environments having a substantial magnetic environment such as one having substantial surrounding steel structure and in environments where the detector is subjected to significant vibration.
In practical application, particularly in adverse weather environments, proximity switches such as those described above, have proven to be less than effective. For example, in cold weather conditions, such as that found in environments associated with the sport of snow skiing and the use of ski chair lifts (personnel transport systems), a wide variation in temperature may be experienced. Temperatures well below 0.degree. F. and large amounts of moisture in the form of snow and ice may be experienced. Ski lifts typically include a plurality of towers positioned up the side of a slope or a mountain. A steel or iron cable is supported by the towers and moved in a continuous loop fashion by a driving apparatus. Chairs holding the skiers or other persons are suspended from the cable. The individual towers typically have grooved wheels to rotatably support the cable. Should the cable become removed or derailed from the grooves of the wheel(s), a "derope" condition would exist. Such a condition can be a hazard to the people riding on the chairs. Accordingly, it is typically necessary that the ski lift system be immediately shut down to minimize the hazard to which the people are exposed. Proximity detectors are positioned on the towers proximate the cable or rope to detect its proximity or proper position in the grooved wheels which support the cable on the ski lift tower.
Reliable proximity switches have heretofore been unknown for application not only for the ski lift environment but also in other environments. Further, proximity switches heretofore having sufficient detection range and having characteristics such that the metal associated with the ski lift tower and its associated apparatus does not adversely affect operation of the proximity detector or unduly constrain detector positioning are heretofore unknown. A detector system having means to test proximity detectors when installed is also heretofore unknown.
Other known detecting devices which may be of interest include devices disclosed in the following U.S. Pat. Nos.: 3,058,059 (Bockemuehl); 3,504,236 (Takesimoyagawa et al); 3,761,736 (Edge et al) disclosing a capacitive touch proximity detector; 3,048,748 (Carey); 3,377,519 (Stong); 3,176,096 (Marcum); and 3,161,742 (Bagno); and 3,736,445 (Van Sickle).